The production of rigid polyurethane foams from organic polyisocyanates and polyols having on average three or more hydroxyl groups per molecule, has been well established for over thirty years. During this time, the foams have become increasingly important as insulating materials and structural components in the construction, transport and other industries because of their ease of manufacture and outstanding thermal insulation properties.
However, polyurethane foams, like most other organic materials, are readily combustible and, in order to comply with increasingly stringent regulations and codes of practice, flame-retardants are often included in the foam-forming reaction mixture. The flame retardants used are generally of two types, those that are merely physically incorporated into the foam structure and those that become chemically incorporated by taking part in polyurethane formation. The first type of flame retardant includes materials such as tris(2-chloroethyl) phosphate, tris(2,3-dibromopropyl) phosphate and antimony oxides.
When used in an amount to provide a high degree of flame resistance, this type of agent often has an adverse effect on the foam's mechanical properties. The second type of flame-retardant is represented by various phosphorus-containing polyols and chlorendic acid derivatives. These agents can be expensive and often have high viscosities. U.S. Pat. No. 3,297,597 describes certain nitrogen-containing polyols which are said to provide rigid polyurethane foam characterised by greater ease of fire retardancy and good dimensional strength when extraneous fire retardants are employed. The nitrogen-containing polyols are oxyalkylated Mannich condensation products of a phenolic compound with formaldehyde and an alkanolamine. A preferred phenolic compound seems to be phenol itself which has three active positions on the aromatic ring at which reaction can occur.
It has now been found that certain Mannich bases derived from 2,4-dibromophenol are useful materials from which rigid foam polyols may be prepared.